A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga 2 O 3 epilayers grown at low temperature (650°C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called ε phase; i.e. they exhibited hexagonal P6 3 mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga 2 O 3 layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5-10 nm size. Each domain has orthorhombic structure with Pna2 1 space group symmetry, referred to as κ-Ga 2 O 3. Further XRD analysis carried out on thicker samples (9-10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains togetherif the domain size falls below the actual resolution of the probing techniquescan be misinterpreted as the disordered structure with its P6 3 mc space group symmetry usually referred to as ε-Ga 2 O 3 in the current literature. The crystal structure of these Ga 2 O 3 layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zigzag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna2 1 space group and previous observations.

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